"Cyberspace is a metaphor that allows us to grasp this place where since about the time of the second world war we've increasingly done so many of the things that we think of as civilization."
- William Gibson

Small parasites that cause muscles to twitch in sequence for strength.

This is another in the amplified muscle category; but it is interesting that these creatures (presumably bioengineered) work your muscles in the same way that those ab workout belts were supposed to do. They build strength by constantly working your muscles.

On a previous visit to the mod parlor, two years ago, he had paid to have a bunch of 'sites implanted in his muscles-- little critters, too small to see or feel, that twitched Bud's muscle fibers electrically according to a program that was supposed to maximize bulk. Combined with the testosterone pump embedded in his forearm, it was like working out in a gym night and day, except you didn't have to actually do anything and you never got sweaty. The only drawback was that all the little twitches made him kind of tense and jerky. He'd gotten used to it, but it still made him a little hinky on those skates, especially when he was doing a hundred clicks an hour through a crowded street.

There are several other ways to do this; you might also go for a refurbished nervous system (see the protagonist in The Stars My Destination) or you might want to get a muscle graft like the joeboys (see Neuromancer).